1. Field of the Invention
This invention pertains generally to surface-enhanced Raman spectroscopy (SERS) sensors, and more particularly to a SERS sensor for arsenic sensing and its method of fabrication.
2. Description of Related Art
Techniques for directing the assembly of metal or semiconductor quantum dots into superstructures have been pursued over the years. Few studies have addressed the organization of one-dimensional nanoscale building blocks such as nanotubes, nanowires, and nanorods into ordered structures except for the 3-dimensional spontaneous superlattice formation of nanorods made from certain materials. On the other hand, Kim, F. et al., “Langmuir-Blodgett Nanorod Assembly”, J. Am. Chem. Soc. 123, 4386-4389 (2001), incorporated herein by reference, describes a method for fabricating a 2-dimensional monolayer assembly of BaCrO4 nanorods using the Langmuir-Blodgett technique.
Various researchers have successfully prepared Langmuir-Blodgett films of spherical nanoparticles such as Ag, Au, and CdS. Typically, the surface of the nanocrystals are functionalized by organic molecules (usually long alkyl chains) in order to prevent particle aggregation and also to ensure the floating of the nanoparticles on the subphase surface (usually water). The nanoparticles are then dispersed in organic solvents such as toluene, and this solution is spread drop-wise onto the subphase surface. The nanoparticles form a monolayer on the water-air interface, which is slowly compressed. This monolayer can be transferred during the compression using either horizontal or vertical liftoff to substrates such as TEM grid or Si wafer to be inspected under electron and optical microscopes. For spherical nanoparticles, the particles form a gas phase at low densities, where the monolayer is highly compressible without significant increase in the surface pressure. Depending on the particle size, the length of the capping ligand, and the surface pressure, various microscopic structures of islands, wires, and rings composed of the nanoparticles can be formed. As the monolayer is compressed, the nanoparticles start to form a condensed phase, usually a hexagonally close packed structure due to the isotropic inter-particle interactions.
Nanoscale science, however, is about assembling matter at multiple length scales, from atomic and molecular species to individual nanoscale building blocks such as nanocrystals, nanorods and nanowires, then from these individual nanoscale building blocks to higher-level functional assemblies and systems. This hierarchical process covers length scale of several orders, from Å to micrometer or larger. The past decades have witnessed great progress in the direction of synthesizing nanocrystals of various compositions and sizes. Significant progress has been made in the area of nanowire synthesis and device application. Successful alignment and patterning of nanowires would significantly impact many areas such as nanoscale electronics, optoelectronics and molecular sensing. A grand challenge, however, resides in the hierarchical integration of the nanoscale building blocks into functional assemblies and ultimately to a system.
Unlike the traditional lithographical process where precise placement of certain elements or devices is embedded in the designing process, the precise placement of nanoscale building blocks on the right place with right configuration and with exceedingly high densities represents a daunting task for researchers in this field.
Nanoparticles have attracted a great deal of attention due to their potential applications in optics, electronics, and catalysis. Different methods have been developed to synthesize metallic and semiconductor nanoparticles of different sizes. In the synthesis of new materials based on an ordered assembly of nanoparticles, three significant factors are important in determining the interactions between the nanoparticles and ultimately their superstructures, namely the shape and size distributions of the nanoparticles, and the surface functionality of the nanoparticles. A major motivation for research in this field remains the challenge to understand how ordered or complex structures form by self- or directed-assembly, and how such processes can be monitored/controlled in order to prepare structures with a pre-determined geometry/superstructure.
A prerequisite for nanostructure preparation via the assembly route is the availability of sufficiently stable building blocks that are highly uniform in size and shape. Techniques for directing the assembly of metal or semiconductor quantum dots into novel superstructures have been extensively pursued over the past decades. Impressive accomplishments in the area of self-assembly of metallic silver and gold nanoparticles, semiconductor CdSe and Ag2S quantum dots and spherical nanoparticles have been reported. This is due to the possibility of obtaining these spherical nanoparticles as highly monodispersed and stable products. In spite of the large volume of research on the self-assembly of quantum dots, however, little attention has been devoted to the self-assembly of rod-shaped nanoparticles (nanorods) and particles with other different shapes (prisms, hexagons, cubes). This is partly due to the fact that there is no chemistry available for preparing these highly uniform facetted nanocrystals.
After decades of research, the size control of the metal and semiconductor nanocrystals is now well-established. The deterministic shape control is, however, still in its infancy although recent efforts into nanorod synthesis have resulted in some very exciting progress. In addition, there has been progress toward shape control of II-IV compound nanocrystals, where easy axis (6-fold symmetry) exists within the crystal structure and has profound impact on the resulting nanocrystal growth habits. In general, however, the mechanism of shaped nanocrystal growth, particularly for those metal systems, is still much elusive and currently under hot debate.
Nanocrystal shape control is still a highly empirical process due to the lack of fundamental understanding of the complex growth process with multiple synthetic parameters. One known approach to shape control is to use surfactants during the metal reduction and particle growth. The surfactant has a role to control the crystal shapes by attaching to selected crystal surface during the growth. Of course, the surfactants also stabilize the metal particles and avoid the undesirable aggregation. In this regard, some linear polymers are recently found to be highly effective to control the crystal shapes. For example, polyacrylate, poly-(N-vinyl-2-pyrrolidone) and polyvinyl alcohol have been used to control the metal particle shapes with a reasonable yield. A main advantage of this surfactant/polymer approach for shaped crystal synthesis is the relative large yield and its potential to produce high purity products.
Besides the surfactant approach, one additional important factor that could determine the final crystal shapes is the addition of foreign ions. For example, it has been found that different ions and ionic strength could be used to modulate the copper nanocrystal shapes. It has also been found that a small amount of silver addition is critical for the formation of gold nanorods in an electrochemical process.
Surface-enhanced Raman spectroscopy (SERS) has previously been employed for chemically specific and sensitive detection of molecular monolayers, small biological molecules, trace-level explosives, organic groundwater contaminants, and single molecules. Typical detection signal enhancements are around 109 greater than sensing performed without a metallic substrate.
Therefore, there is a need for a method of assembling monolayers of nanostructures other than spherical nanoparticles. There is also a need for a method of controlling shape synthesis of metal nanostructures and mediating the interaction among these particles to form different 2-dimensional (2D) or 3-dimensional (3D) superstructures. The resultant superstructures are of importance for their tunable collective physical properties (e.g. optical, magnetic and catalytic properties), where inter-object separation, shape and interfacial structure enable the tuning of properties. A further need is increased Raman enhancement for SERS detection with use of nanostructured silver surfaces.